The present invention relates generally to semiconductor devices and, more particularly, to the use of transition metal boride films as diffusion barriers in devices such as gate stacks and digit line stacks.
In some semiconductor memory circuits, word lines, which are formed from a uniformly-thick conductive layer, form both gate electrodes and gate interconnections. Whenever a word line passes over a field-oxide region, it functions as a gate electrode interconnection; whenever the word line passes over a gate dielectric layer overlaying an active region, it functions as a gate electrode.
In early generations of integrated circuits, gate electrodes and electrode interconnections were often etched from a heavily-doped polycrystalline silicon (polysilicon) layer. To achieve increased operational speeds and lower stack heights-in subsequent generations of circuits, it was necessary to decrease the sheet resistance of the conductive layer from which the gates and gate interconnections were formed. Recently, the use of pure metal layers formed from materials are being investigated to enhance the conductivity of the polysilicon transistor gates and gate interconnections. Tungsten (W), for example, is of particular interest because it is relatively inexpensive, has a high melting point, and is compatible with current circuit manufacturing processes. Thus, low pressure chemical vapor deposited (LPCVD) tungsten silicide (WSix) is being investigated in the fabrication of polycide gate structures to form low resistance word lines in semiconductor devices such as dynamic random access memory (DRAM) cells.
As illustrated in FIG. 1A, a wafer includes a semiconductor substrate 10 which may include one or more previously formed layers or active regions. A gate dielectric such as a silicon oxide layer 14 is deposited or grown over the surface of the substrate, and a gate stack 22 is formed over the silicon oxide layer. The gate stack 22 includes a gate polysilicon layer 16 which helps improve the adhesion of a subsequently deposited tungsten silicide film. The gate stack also includes a tungsten silicide layer 18 deposited, for example, by LPCVD over the gate polysilicon layer 16. The polysilicon and tungsten silicide layers 16, 18 are patterned and etched using conventional photo-lithographic techniques to form the polycide gate electrodes. Ion implanted source and drain regions 12 are formed, and the wafer is subjected to an annealing process at an elevated temperature.
WF6 and SiH4 are among the reaction gases typically used during the deposition of the tungsten silicide film 18, and, therefore, fluorine atoms generally are incorporated into the tungsten silicide film 18. When the polycide structure is subsequently annealed at high temperatures, fluorine atoms tend to diffuse through the gate polysilicon 16 into the gate silicon oxide layer 14. The fluorine atoms react with the oxide and break the Si—O bonds to replace the oxygen at those sites. The released oxygen diffuses to the interface of the SiO2 layer 14 and oxidizes the silicon and polysilicon resulting in an increased oxide thickness 20 (FIG. 1B). The additional oxide can cause device degradation, such as a shift in the threshold voltage and a decrease in the saturation current.
Attempts have been made to reduce the diffusion of fluorine into the gate silicon oxide layer by forming a thin film conducting diffusion barrier between the tungsten silicide film 18 and the gate oxide 14. For example, diffusion barriers of materials such as titanium nitride, tantalum nitride and titanium tungsten have been proposed with some success. Nevertheless, room remains for improvement in structures such as gate stacks as well as digit line stacks, among others.